1. Field of the Invention
This invention relates generally to transceiver architecture in a wireless portable communication device. More particularly, the invention relates to a transmit-receive switch architecture providing pre-transmit isolation.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.
In Europe and in other parts of the world, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology. The GSM communication standard has been further developed into what is referred to as the enhanced data rates for GSM evolution, referred to as EDGE, which uses 8-quadrature phase shift keying (8QPSK). In Europe and in many other regions the GSM communication system operates in the 900 MHz “EGSM900” band and the 1800 MHz “DCS1800” band, while in the Americas it operates in the 850 MHz “GSM850” band and the 1900 MHz “PCS1900” band. Each of the GSM variants uses different transmit and receive frequencies.
For efficiency of deployment, in some instances it is desirable to provide a single communication device that can be used in more than one communication system. These so called “multi-mode” or “multi-band” communication devices are capable of providing communications access in two or more communications systems (multi-mode), or two or more bands (multi-band). For example, in the GSM communications system, some communications devices are capable of operating in the GSM850, EGSM900, DCS1800 and PCS1900 frequency bands. Even though the PCS1900 transmit band overlaps the DCS1800 receive band, and the GSM850 receive band overlaps the EGSM900 transmit band, these communications devices can provide the capability to operate in all of these bands because they only operate in one band at any given time.
Unfortunately however, due to the frequency overlap between the PCS1900 transmit band and the DCS1800 receive band, there is an operating situation in which leakage from the transmit section in the PCS1900 band may leak through the receive section of the DCS1800 band, and in which leakage from the transmit section in the EGSM900 band may leak through the receive section of the GSM850 band. The operating condition arises because the GSM transmit time mask specification requires stringent adherence to power output limitations. For example, during what is referred to as a “pre-transmit time” a transmit voltage controlled oscillator (TX VCO) associated with the DCS1800/PCS1900 transmit section of the communication device is activated to stabilize frequency prior to transmitting, but the communication device is not permitted to transmit until a prescribed time. Specifically, the level of any emitted power must be below a specified limit during the pre-transmit time. To prevent any transmit power from being emitted during this pre-transmit time, one or more transmit/receive switches in the communication device are maintained in a receive position, thus preventing any significant transmit power from being emitted by the transmit circuitry in the communication device during the pre-transmit time. Unfortunately, because the PCS1900 transmit band overlaps the DCS1800 receive band, power from the TX VCO may leak through the DCS1800 receive band circuitry to the antenna, particularly through the surface acoustic wave (SAW) filter associated with the DCS1800 receive circuitry.
FIG. 1 is a schematic diagram illustrating an exemplary front end module (FEM) 10 of a communication device. The front end module 10 comprises an antenna 11 coupled to a diplexer 12. The diplexer separates frequency bands and provides, in this example, GSM850/EGSM900 transmit/receive signals via connection 14 and the DCS1800/PCS1900 transmit/receive signals via connection 16. The front end module 10 illustrates an architecture that combines GSM850, EGSM900, DCS1800, and PCS1900 (also considered within the GSM framework) communication bands on a single portable communication device. This architecture is also referred to as “quadband.” For simplicity of description, only the DCS1800/PCS1900 frequency bands will be discussed. The signal on connection 16 is coupled to a transmit-receive switch 18. The transmit-receive switch 18 can be, for example, fabricated using a gallium arsenide (GaAs) field effect transitor (FET) or any other switch. The transmit-receive switch 18 determines whether a signal received by the antenna 11 will be delivered to receive circuitry or whether a transmit signal will be delivered to the antenna 11 from the transmitter circuitry.
In the receive path, the transmit-receive switch 18 is coupled via connection 38 to a pair of surface acoustic wave (SAW) filters 41 and 42. The surface acoustic wave filter 41 is tuned to receive signals in the DCS1800 communication band while the surface acoustic wave filter 42 is tuned to receive signals in the PCS1900 communication band. In addition, a transmit filter 44 allows the passage of DCS1800 and PCS1900 transmit signals from the power amplifier 49 via connection 48. The surface acoustic wave filter 41 should present a high impedance in the band in which the surface acoustic wave filter 42 operates. Similarly, the surface acoustic wave filter 42 should present a high impedance in the band in which the surface acoustic wave filter 41 operates. This impedance condition may be met in a variety of ways as known in the art.
A transceiver 37 includes a transmit voltage control oscillator (TX VCO) 36 for the GSM850/EGSM900 transmit bands and a TX VCO 52 for the DCS1800/PCS1900 transmit bands. The TX VCO 52 is coupled to the transmit power amplifier 49 via connection 51. The transceiver 37 also includes a low noise amplifier (LNA) 33 for the GSM850 receive band coupled to the surface acoustic wave filter 27 via connection 29, and an LNA 34 for the EGSM900 receive band coupled to the surface acoustic wave filter 26 via connection 28. The transceiver 37 also includes an LNA 54 coupled to the surface acoustic wave filter 41 in the DCS1800 receive band via connection 46, and an LNA 55 coupled to the surface acoustic wave filter 42 in the PCS1900 receive band via connection 47.
The following description will be directed to the DCS1800/PCS1900 bands, but is also applicable to the GSM850/EGSM900 bands. When communicating using time division duplex (TDD) or time division multiple access (TDMA), as used in the GSM communication methodology, there is a portion of the communication time, referred to as the “pretransmit” time, during which the switch 18 remains in the receive position, as shown in FIG. 1, and during which time the TX VCO 52 is activated to power-up and stabilize prior to transmitting. During this pre-transmit time period, and because the PCS1900 transmit band overlaps the DCS1800 receive band, a PCS1900 transmit signal emitted from the TX VCO 52 may leak through the DCS1800 receive path, through the surface acoustic wave filter 41, as shown using reference numeral 60. This leakage path 60 occurs due to the TX VCO 52 being active, and being in close proximity to the receive port 46 of the low noise amplifier 54. Further, leakage from the TX VCO 52 may propagate to other portions of the transceiver 37. This transmit signal leakage through the receive path to the antenna 11 may cause the portable communication device to violate the allowed GSM transmit time mask.
FIG. 2 is a graphical illustration 70 showing an exemplary transmit power curve of a portable communication device operating in the GSM communication environment. The horizontal axis 71 represents time and the vertical axis 72 represents transmit power. The GSM communication system transmits power in what are referred to as “transmit bursts” which occur during carefully controlled time periods. The curve 76 illustrates the transmit power output of the antenna 11 of FIG. 1. The mask 74 represents the GSM transmit spectrum within which the transmit power curve 76 must remain.
During a pre-transmit time, illustrated using reference numeral 77, the TX VCO 52 is on, while the switch 18 (FIG. 1) remains in a receive mode to attempt to prevent transmit power from reaching the antenna 11 (FIG. 1). During this time period 77 the TX VCO 52 (FIG. 1) is on, but is not allowed to transmit. However, as mentioned above, transmit power may leak through the receive path as described above and may cause a portable communication device to violate the GSM transmit spectrum mask 74.
Prior solutions, which isolate the DCS1800 receive circuitry from the PCS1900 transmit circuitry during pre-transmit and transmit time (and which isolate the GSM850 receive circuitry from the EGSM900 transmit circuitry), include additional switches to select the different receive band ports. Unfortunately, additional switches raise the cost and the complexity of the communication device.
Therefore, it would be desirable to efficiently reduce or eliminate any radio frequency (RF) power emitted by a communication device through the receive circuitry.